LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Observation of an Excited B-c(+) State. Phys. Rev. Lett., 122(23), 232001–10pp.
Abstract: Using pp collision data corresponding to an integrated luminosity of 8.5 fb(-1) recorded by the LHCb experiment at center-of-mass energies of root s = 7, 8, and 13 TeV, the observation of an excited B-c(+) state in the B-c(+)pi(+)pi(-) invariant-mass spectrum is reported. The observed peak has a mass of 6841.2 +/- 0.6(stat) +/- 0.1(syst) +/- 0.8(B-c(+)) MeV/c(2), where the last uncertainty is due to the limited knowledge of the B-c(+) mass. It is consistent with expectations of the B-c*(2(3)S(1))(+) state reconstructed without the low-energy photon from the B-c*(1(3)S(1))(+) -> B-c(+)gamma decay following B-c*(2(3)S(1))(+) -> B-c*(1(3)S(1))(+)pi(+)pi(-). A second state is seen with a global (local) statistical significance of 2.2 sigma (3.2 sigma) and a mass of 6872.1 +/- 1.3(stat) +/- 0.1(syst) +/- 0.8(B-c(+)) MeV/c(2), and is consistent with the B-c(2(1)S(0))(+) state. These mass measurements are the most precise to date.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Measurement of the Mass Difference Between Neutral Charm-Meson Eigenstates. Phys. Rev. Lett., 122(23), 231802–10pp.
Abstract: We report a measurement of the mass difference between neutral charm-meson eigenstates using a novel approach that enhances sensitivity to this parameter. We use 2.3 x 10(6) D-0 -> K-S(0)pi(+)pi(-) decays reconstructed in proton-proton collisions collected by the LHCb experiment in 2011 and 2012. Allowing for CP violation in mixing and in the interference between mixing and decay, we measure the CP-averaged normalized mass difference x(cp)= [2.7 +/- 1.6(stat) +/- 0.4(syst)] x 10(-3) and the CP-violating parameter Delta x = [-0.53 +/- 0.70(stat) +/- 0.22(syst)] x 10(-3). The results are consistent with CP symmetry. These determinations are the most precise from a single experiment and, combined with current world-average results, yield the fast evidence that the masses of the neutral charm-meson eigenstates differ.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Combination of Searches for Invisible Higgs Boson Decays with the ATLAS Experiment. Phys. Rev. Lett., 122(23), 231801–20pp.
Abstract: Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H -> invisible decays where H is produced according to the standard model via vector boson fusion, Z(ll)H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb(-1) of pp collisions at a center-of-mass energy of root s = 13 TeV at the LHC. In combination with the results at root s = 7 and 8 TeV, an exclusion limit on the H -> invisible branching ratio of 0.26(0.17(-0.05)(+0.07)) at 95% confidence level is observed (expected).
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Modelling radiation damage to pixel sensors in the ATLAS detector. J. Instrum., 14, P06012–52pp.
Abstract: Silicon pixel detectors are at the core of the current and planned upgrade of the ATLAS experiment at the LHC. Given their close proximity to the interaction point, these detectors will be exposed to an unprecedented amount of radiation over their lifetime. The current pixel detector will receive damage from non-ionizing radiation in excess of 10(15) 1 MeV n(eq)/cm(2), while the pixel detector designed for the high-luminosity LHC must cope with an order of magnitude larger fluence. This paper presents a digitization model incorporating effects of radiation damage to the pixel sensors. The model is described in detail and predictions for the charge collection efficiency and Lorentz angle are compared with collision data collected between 2015 and 2017 (<= 10(15) 1 MeV n(eq)/cm(2)).
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Measurement of W +/- Z production cross sections and gauge boson polarisation in pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 79(6), 535–34pp.
Abstract: This paper presents measurements of W +/- Z production cross sections in pp collisions at a centre-of-mass energy of 13TeV. The data were collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 36.1fb-1. The W +/- Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is sigma W +/- Zfid.=63.7fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of 61.5-1.3+1.4fb. Cross sections for W+Z and W-Z production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.
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